This invention relates to an apparatus and method for measuring rheological properties of a material. Particularly, this invention relates to measuring Theological properties of a material including viscosity, surface tension, elasticity, relaxation time, association time, and yield stress for liquids, solutions, suspensions, and melts.
Extensional flows are common in most industrial processes but are often poorly understood. Fluids are infrequently well-characterized in their extension. The concept of extensional rheometry is analogous to that of torsional shear rheometry. In a shear test, a small quantity of fluid is sandwiched between two plates. One plate is either rotated at a constant rate, or oscillated at a fixed frequency. By measuring the torque required to maintain the rotation rate, or the delay in the applied torque compared to the forced oscillation, one can compute a shear viscosity as a function of shear rate or oscillation frequency.
Instead of obtaining the shear viscosity and other related parameters by applying a known force (i.e., stress) in shear and measuring the resulting displacement or strain, the same procedure can be performed in extension (i.e., tension). Hence the extensional Theological properties bear the same relationship to their shear counterparts as the Young""s modulus does to the shear modulus in elastic solids. From the filling of shampoo bottles to the manufacture of artificial fibers and the coating of rollers in the printing industry, there is invariably an extensional kinematic component. Extensional kinematics always arise in free surface flows (e.g., in jets, fibers, and sheet drawing processes), or where there is a squeezing mechanism or streamline acceleration. However, most viscometric methods available today for rigorously analyzing fluid properties rely on shear rheometry. Materials that exhibit non-Newtonian material properties, such as a non-linear dependence of viscosity with deformation, typically show complex behavior when they are stretched in a flow, as opposed to sheared. Consequently, shear characterization alone is usually insufficient to fully determine a material""s response to the complex flows typically found in industrial processing. Since polymer solutions, melts, and suspensions can have markedly different shear and extensional behavior, this approach can lead to identification of highly misleading parameter values. See, Barnes et al., An Introduction to Rheology (Elsevier, Amsterdam, 1989). While current approaches, such as capillary rheometers or falling ball viscometers, provide some indication as to the apparent extensional behavior of materials, these approaches yield data that index, or rank materials, rather than provide absolute quantitative parameters. It is very difficult to follow these approaches and obtain results that are independent of the experimental configuration. In addition, the thermophysical behavior of the fluid in a stretching flow field exposed to ambient conditions (where curing, gelation or mass or heat transfer may occur) may in itself be of interest (for instance in fiber spinning applications). Curing, vitrification, and crystallization all are strongly influenced by the flow field and can be greatly enhanced or retarded in the presence of extensional flows. A technique that can measure relevant material properties for such processes would therefore be invaluable.
Currently there are few other commercially available methods for obtaining data on the extensional behavior of complex fluids (e.g., colloids, adhesives, paints, foods, consumer products, and melts). This paucity of instrumentation is despite increasing academic and industrial interest in measuring the extensional viscosity of a material. Additionally, existing instrument designs are bulky, complex, and expensive. One example of existing commercial extensional rheometers is the RME(trademark) melts rheometer. See, U.S. Pat. No. 3,640,127 and Rheol. Acta., 33, 1-21 (1994). In this instrument, grooved belts stretch a rectangular polymer specimen at temperatures above the polymer""s melt temperature while monitoring the force exerted by the specimen.
Another technique is based on atomizing liquids to a measurable particle size in a gas flow. See, U.S. Pat. No. 5,559,284. This technique uses empirical expressions based on known physical properties of the test liquid and atomization conditions to determine the elongation viscosity and surface tension. A small quantity of fluid is stretched between two plates. See, USH0000976 (1991). The stretching is achieved by lowering the bottom plate, which sits on an air cylinder piston. A camera is used to photograph the fluid ligament as it stretches.
A different technique involves extruding a test fluid through a capillary downward into a host fluid, the latter material having a lower density and immiscibility with the test fluid. See, USH0000053 (1986). The drop of test fluid elongates and eventually breaks from the capillary. The extensional viscosity is assessed from photographs of the elongating test fluid drop.
The technique of lubricated flow was used in a converging slit die to determine the extensional viscosity of polymer melts. See, U.S. Pat. No. 5,357,784. In a similar approach, a sample flow is forced through an orifice, thereby affecting an extensional flow. See, U.S. Pat. No. 5,900,539. The extensional viscosity is determined from the flow rate and pressure drop across the orifice.
In U.S. Pat. No. 3,693,425, a filament is wound around a drum while measuring the force on the fixed end of the system. In WO 00728321, a fiber is wound around two drums, one of which measures the torque required to maintain the stretching rate.
Instruments were also developed for extensional characterization of fluids. These systems are based on the filament stretching designs in which a small quantity of fluid is stretched between two plates. See, Vinogradov et al., J. Polym. Sci., Part A-2 8, 1-17 (1970) and Mxc3xcnstedt et al., Rheo. Acta 20, 211-221 (1981). Data is extracted from these devices through quantitative observations of the evolution of a thin fluid filament under the combined action of viscous, elastic, and capillary forces. Usually both the tension in the filament and the evolution of the diameter are captured. In the original xe2x80x9cfalling platexe2x80x9d configuration, the sample is subject to a constant load that is imparted by a known weight attached to one of the endplates constraining the sample. See, Matta et al., Journal of Non-Newtonian Fluid Mechanics 35, 215-229 (1990). In a filament stretching device, a known exponential displacement profile is imposed and quantitative measurements of the tensile force along the fluid column, Fz(t), and midpoint filament radius, Rmid(t), are followed in time. See, Tirtaatmadja et al., Journal of Rheology 36, 277-284 (1993) and Spiegelberg et al., J. Non-Newtonian Fluid Mech. 64, 229-67 (1996). Extensional flow is of primary interest to industry because almost all processing conditions in manufacturing involve a component of extensional flow. For instance, pumping, fiber spinning, extrusion, molding, and filling processes all involve stretching kinematics. The behavior of all but the simplest materials in such a flow, however, is markedly different from that predicted from knowledge of the shear rheology. Consequently, for process improvement, manufacturing control, and the development and design of materials and components, knowledge of the extensional properties can be critical.
In more recently developed systems, a xe2x80x9cneckedxe2x80x9d liquid bridge configuration is generated by rapidly separating two cylindrical plates a small distance, and the evolution of the midpoint radius, Rmid(t), is subsequently followed in time with a non-contact micrometer. Regression of the data leads to the calculation of a Newtonian viscosity or a single mode relaxation time. See, Bazilevsky et al., xe2x80x9cLiquid filament Microrheometer and Some of Its Applications,xe2x80x9d D. R. Oliver, ed., Proceedings of the Third European Rheology Conference (Elsevier, 1990), Entov et al., J. Non-Newtonian Fluid Mech. 72, 31-53 (1997), and Stelter et al., Journal of Rheology 44, 595-616 (2000).
The apparatus and method described herein generally relate to a capillary breakup rheometer and measurement of rheological properties of a wide spectrum of fluids not accessible with existing instrumentation.
In one aspect, the invention relates to an apparatus for measuring an extensional rheological property of a fluid. The apparatus includes opposed surfaces defining a sample site disposed therebetween, a housing about the sample site, a light source, and a light detector. The opposed surfaces are adapted for axial motion to vary the vertical dimension of the sample site. The housing about the sample site permits specification of ambient conditions. The light source directs a light beam at the sample site. The light detector senses light passing through the sample site.
In another aspect, the invention relates to an apparatus for measuring an extensional rheological property of a fluid. The apparatus includes opposed surfaces defining a sample site disposed therebetween, a plunger assembly, a light source, and a light detector. The opposed surfaces are adapted for axial motion to vary the vertical dimension of the sample site. The plunger assembly allows variation of the vertical dimension of the sample site and variation of the rate of axial movement of at least one of the opposed surfaces. The light source directs a light beam at the sample site. The light detector senses light passing through the sample site.
In yet another aspect, the invention relates to an apparatus for measuring an extensional rheological property of a fluid. The apparatus includes opposed surfaces defining a sample site disposed therebetween, a light source, a light detector, and a data analysis unit. The opposed surfaces are adapted for axial motion to vary the vertical dimension of the sample site. The light source directs a light beam at the sample site. The light detector senses light passing through the sample site. The data analysis unit converts data from the light detector to an extensional rheological property of the fluid sample.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following drawings, description, and claims.